1. Field of the Invention
The invention relates, in general, to a radio frequency receiver and, in particular to a radio frequency signal receiver with adequate gain control for adjusting the gain of the mixer and/or the channel selection filter.
2. Description of the Related Art
In typical radio frequency receiver with IFAGC (Intermediate Frequency Automatic Gain Control), the AGC operation follows received signal strength after a filter to adjust front-end gain of the receiver. Unfortunately, the received strong adjacent—channel interference before a filter would distort the filter and subsequent stages if it is not properly attenuated at the front-end stage. Therefore, RFAGC (Radio Frequency Automatic Gain Control) is used to solve this problem. By following the strength of received signal and interference, the gain control operation of the RFAGC can adaptively track the magnitude variations of interference to avoid distortion. However, in general RFAGC gain control operation, only the gain of LNA (Low Noise Amplifier) is adjusted to attenuate interference, which would introduce a trade-off between noise figure and dynamic range. For example, if LNA gain decreases excessively, output noise floor will raise significantly, which results in noise figure degradation.
FIG. 1 is a block diagram of one embodiment of a conventional radio frequency signal receiver with IFAGC. A low noise amplifier (LNA) 11 receives and amplifies radio frequency (RF) signals. The mixer 13 down converts the amplified RF signals to in-phase (I) signals and quadrature-phase (Q) signals based on the output signal of the local oscillator (LO) 12. The channel selection filter 14 receives and filters the signals I and Q to develop the desired channel signals. The limiting amplifier 15 receives and amplifies the output signals from the channel selection filter 14. A received signal strength indicator (RSSI) module 17 further measures the signal quality of the amplified output signals and provides a RSSI value. The automatic gain control (AGC) unit 16 adjusts the gain of LNA 11 based on the RSSI value of the RSSI module 17 and feedbacks to the LNA 11. However, according to the operation of the blocks in FIG. 1, the received adjacent interference after the channel selection filter would be filtered out and would not affect the RSSI value. Thus, strong adjacent interference will distort the channel filter and generate non-linear spurs.
FIG. 2 is a block diagram of another embodiment of a conventional radio frequency signal receiver with RFAGC. Compared with the receiver in FIG. 1, the main difference is that the input signal of the AGC 16 is the output signal from LNA 11 instead of from the RSSI 17. By following the strength of received signal and interference, the gain control operation of the RFAGC can track the magnitude variations of interference at LNA output to avoid distortion in the following stage. This conventional architecture can reduce the non-linear spurs, but the unwanted impact would still occur. In FIG. 2, only the gain of LNA (Low Noise Amplifier) is adjusted to attenuate interference and to reduce the distortion, which would introduce a trade-off between the noise figure and dynamic range of the receiver. For example, if LNA gain degrades excessively, it would cause a significant rise in noise floor to deteriorate the noise figure. Please refer to FIG. 3, which is a spectrum diagram of wanted signals and adjacent channel interference after processing by LNA, mixer, channel selection filter, and AGC unit in FIG. 2. Signal represents a signal of a desired channel and Adj. represents an adjacent channel interference. In addition, S1 shows signals at the input of the LNA 11, S2 shows signals at the output of the mixer 13 and the input of the channel selection filter 14, and S3 shows signals at the output of channel selection filter 14 and the input of the limiting amplifier 15. In S1, the strength of Signal is −50 dBm, and the strength of Adj. is −30 dBm. In S2, i.e. signals Signal and Adj. have been processed by LNA 11 and mixer 13, the strength of Signal and Adj. respectively increases to −20 dBm and 0 dBm. Next, signals Signal and Adj. are processed by the channel selection filter 14 with a fixed gain 10 dB, which has insufficient dynamic range to process so strong interference well. Thus, non-linear spurs as labeled as 31 and 32 in FIG. 3 will occur. In the operation of the receiver in FIG. 2, one way to reduce the non-linear spurs is to decrease more gain of LNA 11; however, in that way, signal-to-noise ratio of the radio frequency signal receiver will deteriorate significantly.